Combustion engines of motor vehicles comprise at least one combustion chamber formed between a cylinder, a piston sliding in the cylinder and a cylinder head facing the piston. Such a cylinder head bears at least one valve and at least one spark plug for petrol engines or a glow plug for diesel engines. Such a cylinder head of the engine can also be provided with a pressure sensor to know the value of the pressure inside each cylinder with the aim of better controlling the progression of the combustion within the engine. This information is used to regulate injection of fuel into each of the cylinders with more precision. The fuel consumption of the engine can thus be optimized and polluting emissions reduced.
Application WO2010/26316, which is incorporated by reference describes this type of pressure sensor integrated into the cylinder head within the context of diesel engines, petrol engines or the like, it being possible for said sensor to be completely or partly integrated into the glow plug, the injector or the like.
It is also known practice to drill a housing into the cylinder head in order to insert the pressure sensor therein and give it access to the combustion chamber. Thus, the pressure sensor communicates with the combustion chamber through a small tubular cavity. Such a tubular cavity promotes the formation of oscillatory harmonic acoustic waves. Such waves are due to the propagation of the pressure waves in the tube and disturb the reading of the actual pressure by the sensor. Specifically, the wave propagates in the tube and is reflected by striking the measuring element of the sensor, which results in cavitation phenomena. These acoustic waves, through their impacts on the aforesaid measuring element, are then superimposed onto the actual pressure of the combustion, which creates a noise on the measurement.
In addition, the chemical reaction of the combustion of an engine produces various residues which are generally referred to as “soot”. This soot is propagated by the turbulence of the combustion and can be deposited on all the metallic surfaces of the chamber, leading to coking of the engine. Therefore, the soot is also deposited on the measuring sensor and, in particular, on the part of the sensor positioned facing the combustion chamber. This deposit promotes the creation of a crust which changes or disturbs the measuring properties of the sensor. In particular, such a crust, because it is rigid, alters the measuring sensitivity of the sensor since its sensitive element will then be less exposed to the effect of the pressure retained by the crust.
Moreover, the presence of a black crust on the measuring sensor changes the effect of the temperature of the combustion on the end piece of the sensor, which leads to thermal disturbances on the sensor and causes deformations, stress, strains, etc. The sensor therefore becomes more sensitive to thermal shocks and to pressure variations during the combustion cycles, which over time has an impact on the reading precision and the service life of the sensor.